Conventional security documents such as currency, stock and bond certificates, birth and death certificates, land titles and the like are typically made of paper. However, paper, even the more durable security paper, is not particularly durable. Since polymeric materials may be more resistant to damage caused by handling, environmental exposure and water, they may be used to replace paper for security document applications.
Polymeric documents offer several benefits over their paper counterparts. In particular, polymeric security documents can offer greatly increased durability and resistance to counterfeiting through the incorporation of security features. Holograms and other diffractive or interference optical layers are often applied to the surface of banknotes, as are optically variable and intaglio inks. The polymeric security documents may also have certain physical properties that are similar to the more commonly used paper banknotes, such as tactile feel, strength, tear resistance, handling, folding, and crumple resistance.
However, the capture of the image-forming ink on polymeric substrates presents a technical challenge because plastic film is substantially impervious to liquids. For example, US 2003/0232210 A1, incorporated herein by reference, describes security document substrates made from oriented foam polyolefin films. An ink receptive surface is provided on the oriented polyolefin foam to capture and retain an image forming ink. The ink receptive surface is prepared by corona or flame treating the surface of the oriented foam polymeric film, by applying to the oriented foam polymeric film a suitable ink receptive primer coating, or by laminating or coextruding onto the oriented foam polymeric film an ink receptive polymer film.
Typically, ink receptive coatings are made of highly filled binder compositions where the filler content usually is greater by weight than the polymer binder. Such high filler concentration is needed to create micro porous structures in the coating, where the ink is absorbed into the pores by capillary action. For synthetic films made of polyolefin materials, coatings with high filler content have very poor adhesion to the substrate. Therefore, there is a need to develop ink receptive coatings for substrates that have low filler content with good ink absorption and good adhesion to the substrate.